Thank you for the additional information. I really appreciate it.
First and most importantly, I am so sorry for your loss. It's always heart breaking when you lose a companion, and I think that heartbreak is intensified when the cause of the death is still in question.
There are autopsies done on animals - they're called 'necropsies', and your regular vet can do one, or you can request an independent necropsy be done at a different clinic to determine her cause of death. This obviously can only be done if the death is fresh and the animal hasn't been sent for cremation.
As for the cause of death, without being able to see her and do any testing or knowing her complete medical history, it's hard to say for sure what caused her sudden death, however from my experience in the veterinary world combined with your description of her symptoms right before her death and the fact she was recently in heat, I'm tempted to believe she had a pyometra that may have ruptured.
A pyometra is an infected uterus and we see this most commonly within a few weeks after a heat cycle, and on older dogs. Often, we see something called an 'open pyometra' where the dog has her heat cycle and then within a few weeks, the dog is lethargic, anorexic and has a foul smelling discharge from her vulva. These are dogs that generaly get to be on some antibiotics for a few days and then are spayed. However, there are also 'closed pyometras' where there may not be any tell-tale signs and no discharge, so they're often missed. Since you had only had her for a short time, she may have shown signs that she wasn't feeling well...but it's also possible it happened so suddenly that there was no sign. A necropsy will tell you more, and will likely give you some closure as to her cause of death.
I wish I were able to tell you what happened to your sweet girl, but I'm hopeful that the necropsy will give you the answers you're looking for.
Again, I am so sorry for your loss.